Introduce shellsplit.cmd command line parsing subroutine.
Dealing with the limitations of cmd.exe's command-line parsing forces users to use workarounds, and corner cases still remain.
By implementing our own command line splitting, we conceal the complexity and ensure maintainability.
Makefile macro definition and opt-dir list now free from quoting.
now can use like
```
configure.bat --with-opt-dir=c:/src/zlib;c:/src/libffi CC="cl -std:c11" DEFS=-DOPT_THREADED_CODE=2
```
